The present invention relates to a pre-assembled two-stage reaction device for vacuum brake boosters comprising a vacuum chamber in which a constant pressure prevails and a working chamber in which different pressures prevail, the chambers being separated by a movable wall; a control valve actuatable by a brake pedal and controlling the differentials of pressure acting on the movable wall; a push rod acting on a master cylinder; and reaction levers which are in engagement with the movable wall at a first location and with the push rod and a reaction plate connected thereto at a second location. The first location is disposed radially farther out than the second location, and the reaction levers are acted upon by a cup spring at a third location, with the cup spring being positioned on the side of the reaction levers close to the vacuum chamber and with the third location having a greater radius than the first location. In addition, a spring plate is positioned between the cup and the reaction levers, the spring plate having a circumferential collar with flattened portions for the reaction levers on the side adjacent the reaction levers.
From U.S. Pat. No. 3,102,453, a vacuum brake booster is already known which comprises a constant-subatmospheric pressure chamber and a working chamber in which different pressures prevail, as well as a movable wall separating the chambers and being mounted on a plunger, and a control valve which is actuatable by a brake pedal. The control valve controls the differentials of pressure acting on the movable wall. The end of the plunger adjacent the control valve includes a pin on which a plate is urged. Between the plate and the control valve, three reaction levers are positioned which on the one side bear against the movable wall radially outwardly and against a spring radially inwardly, and on the other side bear against the plate. In this arrangement, the spring is located in an opening, close to the vacuum chamber, in the valve piston of the control valve. The biasing force of the spring provides for what is termed "two-stage action" which means the retardation of the reaction force acting on the brake pedal. The magnitude of the "two-stage action" is dependent upon the strength of the reaction-delaying spring and the distances between the points of force transmission.
In the known vacuum brake booster with a two-stage reaction device, the outer ends of the reaction levers abut directly on a component associated with the control valve and rigidly connected to the movable wall. As a result, as has been explained hereinabove, the reaction levers act on the movable wall. This lever device cannot be used in devices which include a control housing made of thermoplastic material, since the latter material does not have the strength to enable the levers to be supported directly on the control housing. In addition, in this known construction, the space available is limited, which entails difficulties during the assembly.
Similar vacuum brake boosters are known from German Pat. DE-OS No. 2,822,101 and DE-OS No. 2,823,784 whose two-stage reaction devices differ from the one in U.S. Pat. No. 3,102,453 basically in that the springs are positioned on the side of the reaction levers adjacent the vacuum chamber and in that the point of engagement of the springs is located in each case outside the point of force transmission at the movable wall or at the control housing component connected thereto. However, also in these known constructions, there are encountered the above-mentioned difficulties with the use of thermoplastic material and the assembly problems.